Gettering vacuum tube



Nav. 30, 1937. ET AL 2,100,746

GETTERING VACUUM TUBE Filed Nov-. 7, 1935 INVENTORS HENRY J. MILLE Y LWOOD W.SCHA M Patented Nov. 30, 1937 f 210mm cnr'mnmo VACUUM TUBE;

Henry J. Miller, Maplewood, N. J., and Elwood W. Schafer, New York, N. ,Y., assignors, by mesne assignments, to Radio Corporation of america, a corporation of Delaware Application November '1, 1935, Serial N0. 48,658

7 Claims. 250-275) UNITED STATES PATENT OFFICE This invention relates to the manufacture of vacuum tubes such as electron discharge devices and the like, and more particularlyto the evacuation of such devices by the aid of chemically active vaporizable clean-up agents or getters which are vaporized or flashed during exhaust to reduce the pressure of the residual gases and z'aporsleft in the device by mechanical pump- It is the usual practice to use as a'getter or clean-up agent an active metal, such as an alkaline earth metal, and to introduce the agent into the device in the form of compounds of the metal from which the metal may be liberated by heating and decomposing the compound after the tube is evacuated by the pumps. Extensive use has been made of alloys of the active alkaline earth metals such as alloys of barium and aluminum or magnesium and aluminum or of all three metals. In these alloys the aluminum protects the active alkaline earthmetals against oxidation until the alloy is decomposed in the discharge tube by heating. Usually these alloys to be introduced into the discharge tube are shaped into pellets and fixed to a getter carrier or getter tab which is then heated at the proper stage of exhaust by high frequency induction heating.

The conventional methods of using these getters have given good results in electron discharge devices having glass envelopes, but the results obtained with them have not been so favorable in the metal tubes in which the envelope consists of iron or a mild steel. The commercial procedure in exhausting a metal tube involves the heating of the shell orenvelope of the metal tube by gas flames to a red heat during exhaust and dispenses with the use of the high frequency heating generally used in connection with the glass envelope tubes. It has been found difiicult to obtain and to maintain in the metal tubes as good a vacuum as is desired, due apparently to the liberation of gases from the metal parts of such a way as to have and to maintain throughout its useful life a better vacuum than is possible with the conventional methods of exhaust now in' use.

Further objects and advantages of this invention will appear from the following specification and detailed description.

In accordance with this invention an alloy of barium and aluminum isv intimately associated with free aluminum preferably by placing the alloy in a-casing or tube of aluminum or in a container of sheet aluminum or foil. The barium aluminum alloy which we prefer to use is friable and is preferably powdered and enclosed in the aluminum container, thus forming a getter capsule. This getter capsule is then mounted on and preferably in close thermal contact with the wall of the iron envelope of the metal tube and at the proper time during the exhaust of the tube is heated to the temperature at which the getter fiashes, preferably by means of a pointed flame.

directed against the outside of the metal enve- Figure 1 shows an electron discharge device with a metal envelope and a getter capsule made in accordance with this invention;

Figure 2 shows a getter capsule constructed in accordance with this invention;

Figure 3 is a detailed view of one method of mounting this getter capsule on a curved metal wall;

Figure 4 is a cross sectional view on an enlarged scale of the wall of an iron envelope which has been gettered according to this invention; and

Figure 5 shows a modified getter support for our novel getter capsule.

In the specific embodiment of this invention illustrated in Figure 1, which shows an application of this invention to metal tubes, we have shown a metal tube comprising an envelope I having an exhaust tube 2; The getter capsule 3, consisting of a short piece of aluminum tubing containing the powdered alloy and 'pinched at the ends, is according to this invention located in any desired position on the side wall or in the top or dome of the envelope. The getter which we prefer to use and with which we have obtained good results is air alloy ofapproximately equal parts of barium and aluminum powdered to pass through a 200 mesh screen and preferably formed into 29 to 30 mg. pellets wrapped in foil oi metallic aluminum to form a getter capsule. One convenient way of making such getter capsules is to pour the powdered alloy into a long cylinder or tube of aluminum with an outside diameter of about .09 of an inch and a wall thickness of about .01 of an inch, and' to tamp the powder firmly in the tube by a ramrod. The tube is then pinched ofi in short sections to form capsules filled with the alloy and of lengths of about onehalf of one inch as shown in Figure 2. The getter capsule maybe attached to the side wall of the envelope by any retaining means such as a strap welded to the wall. These capsules may be flattened by rolling to an outside thickness of .04 to .05 inch and curved to the inner wall of the metal envelope as indicated inFigure 3 to ncunced by the inclusion in the capsule of some of the tube, which may conveniently be of .025 I inch drawn steel, is heated to a bright red heat by gas flames to drive out the occluded gases from the envelope and also to heat the electrodes inside the envelope to degas them. A few seconds after the gas flames have been removed, and while the tube is still at or slightly below a dull red heat, the getter of this invention is flashed by directing against the wall of the tube at the spot where the getter is secured a pointed gas flame which raises the temperature'of a spot about one half inch in diameter on the wall of the tube to near white heat. Weprefer to attain the temperature in about four or five seconds, which can be done by supplying oxygen to the gas flame. This localized heating of the envelope at the point where the getter is secured seems to initiate a reaction in the getter which is apparently exothermic, as the hot spot on the tube quickly increases in brightness, and often attains a temperature which as near as we have been able to determine is about 1400 C. This sudden and marked increase in temperature of the hot spot indicates that the getter has flashed. The reactlonbetween the elements of the getter may sometimes be made more p: o-

oxyg'en'as in some cases better results are obtained by heating the getter capsule in air before It has been found that metal tubes and gettered as above described are remarkably free from gas compared with similar tubes gettered with barium-aluminum alloy flashed in the conventional way on a nickel tab. A tube gettered as above described, when opened shows a bright surface or lining on the inside as indicated in Figure 4, which after exposure to the atmosphere, changes color characteristic of barium, and when chemically tested appears to be predominantly metallic barium. This extended area of active barium on the inner wall of the envelope apparently acts as a keeper to maintain the envelope gas free throughout normal operating life.

There is also found on the inner wall of the iron envelope evacuated in accordance with this invention, at the getter location (5), Figure 4, a closely adherent hard mass of barium aluminum alloy with considerable aluminum, part of which is combined with the iron of the envelope to produce a hard film which appears under a microscope to be analogous to the film on calorized iron articles. The temperature and pressure to which the getter is subjected-makes the evaporation of aluminum probable, and the appearance of the wall indicates aluminum is deposited throughout the inner surface of the envelope. Further, it has been found that fresh barium may, after aging of the vacuum tube, be liberated from the coated spot of the getter location merely by heating the envelope at said location to a red heat, since residual gases accumulated in the tube after the sealing of! may be cleaned up ,by reheating thegetter. When, with a current meter in the anode-cathode circuit of'the tube, theenvelope is reheated, a momentary increase in gas discharge current through the tube is observed indicating the evaporation of additional getter material, followed by a decrease in gas current to mac-- tically zero, indicating a clean up of the residual gases.

The fact that the barium of the alloy of our improved getter' is shielded to a certain extent against contamination by moisture or gas by the protective covering of aluminum, may account forthe greater amounts of available barium, and

hence the improved gas cleaning-up action of our 1, Figure 5, and mounting the tab in the tube,

which is then heated to the flashing temperature of the getter by means of high frequency induction heating.

We claim:

1. A clean up agent adapted for use in producing high vacua which comprises an alloy of aluminum and barium, free aluminum in contact with saidalloy, and iron in contact with said free aluminum.

2. A getter composite comprising a getter of powdered alloy of barium and aluminum, a metal getter support, and a metal adapted to react exothermically with said support interposed ,betweenand in close contact with said getter support and said getter.

3. An evacuated electrical device comprising a sealed iron envelope enclosing cooperating elecv trodes, said iron envelope having on an area of its inner wall an adherent'coating of aluminum combined with the iron and a. coating of at least equal area with a surface which is predominantly barium. 1

4.Processorprodminghighvacuainaniron envelope which comprises placing in good thermal conductive relation to a portion of the wall a powdered friable alloy of barium and aluminum lnintimatecontactwithalayerof aluminuminterposedbetweensaidalloyandsaidwallandin good contact with said portion of said wall, me-- combine part of the aluminum with the iron and minum with said alloy and with the iron, and heating to volatilize at least part of the clean-up material in said agent.

6. Process of producing a high vacuum in an envelope with a getter capsule, of aluminum containing an alloy of aluminum and barium, 'held in contact with iron comprising, mechanically evacuating said envelope, heating said capsule to volatilizing at least part of the alloy by the combined heating action of said iron and the exothermic reaction between the elements of the 7 capsule and said iron.

7. Process of producing a high vacuum. which comprises introducing into an envelope a. cleanup agent consisting of two metals in mutual contact, one being composed predominantly of free aluminum in contact with iron, and the other being an alloy of aluminum and barium, mechanically'evacuating said envelope; and alloying the free aluminum with the aluminum of the alloy and with the iron, and heating to volatilize at least part of the clean-up material in said agent. 10

HENRY J. MILLER. ELWOOD W. SCHAFER. 

